1. Field of the Invention
The present invention relates to a thin-film multi-layer substrate using pins as input/output terminals and to an electronic device.
2. Description of the Related Art
In recent years, thin-film multi-layer substrates have been used in devices and equipment which have small sizes and reduced weights and which employ LSIs of small sizes with many terminals. A thin-film multi-layer substrate has a thin-film structure including an insulating substrate base plate, a plurality of conducting layers and a plurality of insulating layers formed on the substrate base plate, and is suited for accomplishing fine-pitch wiring by mutually connecting the conducting layers by via structures (via holes).
An inorganic material such as alumina or mullite is used as material of a substrate base plate for a thin-film multi-layer substrate. It is also possible to use an organic material such as FR4 as the substrate base plate. The thin-film structure is obtained by laminating a conducting layer such as copper or aluminum and an insulating layer such as polyimide or epoxy, one on the other, the conducting layer being formed as a pattern for supplying electric power and for signal wiring, and the different conducting layers being connected together by via structures.
Pads are formed on the surface of the thin-film structure, and electronic parts such as LSI chips, resistors and capacitors are connected to the pads. Furthermore, the thin-film multi-layer substrate includes pins which form input/output terminals. Like the LSI chip, pins are connected to the pads on the surface of the thin-film structure. FIG. 5 of the attached drawings is a view illustrating a prior art for providing pins on the thin-film structure. In FIG. 5, a thin-film structure 114 on a substrate base plate 112 of a thin-film multi-layer substrate 110 comprises a plurality of conducting layers 118b to 118f and a plurality of insulating layers 120a to 120e. A pin 116 forming an input/output terminal is mounted on a pad 124 on the surface of the thin-film structure 114, and is connected to one of the conducting layers by a via structure 122. The via structure 122 is formed by removing the insulating layer on the surface of the thin-film structure 114 down to a conducting layer to which the connection is to be made. Under the via structure 122 are formed conducting layers 118e and 118f as signal wiring. Thus, the thin-film multi-layer substrate is used with electronic parts such as LSI and the like, as an electric device such as multi-chip module or the like. Such an electronic device is used by mounting it to a printed wiring board or to a motherboard by using pins serving as input/output terminals. The LSI chips and the input/output pins may be mounted to the same surface or to separate (both) surfaces.
Prior arts related to the thin-film multi-layer substrate have been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-283557, No. 6-13755, No. 7-79078 and No. 7-86737. These prior arts, however, do not particularly serve as references for the present invention.
A pin serving as an input/output terminal is attached to a pad on the surface of the thin-film structure, and is connected to one of a plurality of conducting layers by a via structure provided in the thin-film structure. The pad on the surface of the thin-film structure has an area slightly larger than the area of the lower end of the pin, and the pin may be soldered on the pad. On the other hand, the via structure is a fine structure which is usually used for connecting the layers. When a wide area having a pad can be obtained on the surface of the thin-film structure, a multi-layer via structure can be formed to connect a plurality of layers. However, a via structure for connecting the surface of the thin-film structure to a desired conducting layer, is very finely constructed relative to the area of the pad, and a difference between the areas of the pad and the via structure is very large.
Under the pad are arranged not only the via structure but also the conducting layers and the insulating layers of the thin-film structure. Besides, the via structure extends only up to a conducting layer to which the connection is to be made. Under the conducting layer are arranged separate conducting layers, i.e., many signal wires via insulating layers.
When an electronic device including a thin-film multi-layer substrate is to be mounted to a printed wiring board or the like board, the other ends of the pins may be connected to the printed wiring board by soldering. In this case, the electronic device can be neither removed from the printed wiring board nor replaced unless the solder is melted. Nowadays, however, customers are using equipment very frequently, and it is desired that equipment can be repaired or components can be replaced by high-performance components within periods of time as short as possible. It is further desired that replacement can be done without requiring special tools or a particular technology.
For this purpose, a structure is desired in which an electronic device including a thin-film multi-layer substrate is mounted to a socket or to a connector instead of relying upon soldering. In this case, the conduction between the pin and the printed wiring board, which is based upon the contact of the socket with the pin, must be comparable to that accomplished by the soldering. Therefore, a force is imparted to the pin in an undesirable direction at all times, the force being greater by several tens of times than that when the connection is done by soldering. When the thin-film multi-layer substrate receives a load under high-temperature and high-humidity adverse environment, the adhesive force decreases on the interfaces among the insulating layers and the conducting layers as the insulating material absorbs moisture, and satisfactory strength is not reliably maintained for long periods of time. In the conventional thin-film multi-layer substrate, however, efforts have been preferentially made for obtaining fine multi-terminal wirings but not much consideration has been given to the strength of the pins and the via structure.